Automatic volume control



2 1950 R. F. SMELTZER 2,535,325

AUTOMATIC VOLUME CONTROL Filed Feb. 25, 1946 SIGNAL SIGNAL RAYMOND- F.SMELTZER Patented Dec. 26, 1950 AUTOMATIC VOLUME CONTROL Raymond F.Smeltzer, Towson, Md., assignor to Bendix Aviation Corporation, Towson,Md., a corporation of Delaware Application February 25, 1946, Serial No.650,062

3 Claims.

This invention is directed to automatic volume control circuits, and,more specifically, it is directed to A V. C. circuits wherein adidtionalamplification is provided in the A. V. C. network vide an A. V. C.system which eliminates additional I. F. amplifiers in the A. V. C.channel, and thus minimizes the possibility of the introduction ofundesired feedback.

It is a further object of this invention to provide an A. V. C. systemwhich eliminates conventional D. C. amplifiers and their attendantinherent instability.

A further object of this invention is to provide an A. V. C. systemwhich applies the very mini mum of load to the signal-translatingchannels of the systemto which it is connected.

A further object of this invention is to eliminate one or more of theshortcomings of the prior-art arrangements.

5 the reference numeral ill.

The above and further objects and novel features will more fully appearfrom the following detailed description when the same is read inconnection with the accompanying drawings. It

over that incorporated in the normal signal chan- 5 is to be expresslyunderstood, however, that the nels of the receiver intermediatefrequency amdrawings are for purposes of illustration only, and plifiersystem. are not intended as a definition of the limits of Amplified A.V. C., as this type of arrangement the invention, reference for thispurpose being is generally known, has, in the past, been obhad to theappended claims. tained either by further amplification of the in- Inthe drawing termediate frequency signal before application to Figure 1is a schematic diagram of a circuit the A. V. C. rectifier, or byamplification of the constituting an automatic volume control system .C.control voltage produced by the A. V. C. body the i ve i d, Figure 2 isa scherectifier before its application to the controlled a e d a o aPOItiOII 0f the Circuit of stages. The former method, namely, the incorue 1 modified y the addition of means to poration of additional amplifierstage at the i delay the initiation of the automatic volumecontermediate frequency, often introduces instability trol action. dueto the increase in the overall amplification The objects of h nv n r plh of the I. F. system, and usually causes regeneray a system Which pp toe aut t c V01- tion, particularly in receivers of moderately priced ti econtrol bu o a high pedance so rce construction. This regenerationproduces many a n at ve potential sufficient to reduce the gainundesirable effects among which ar di t ti of the receiver to a valuesuitable for the reception due to the non-linearity of the amplificationof of h strongest i n ls p to be ncountered. regenerative amplifiers,and an excessive narrow- This P en is ul fi in the absence of ing of theband width passed by th system d nals above a certain level by theaction of a norto the effectively increased Q of the I. F. channel.melly conducting Control tube. A Combination The latter method ofincreasing the available diode-triode tube y be utilized as e co o A. V.C, voltage, namely that of including stages tube, automatic volumecontrol action being obof D. C. amplification between the A. V. C.rectiteined y the pp a of n y p fi d fier and controlled stage or stagesis generally signal Voltage ved from the ud o detector moresatisfactory, but, due to the inherent instage of the receiver to alocal circuit restricted stability of direct current amplifiers ingeneral, o he control tub a including t diode eethe design andconstruction f s h arrangetion thereof, which rectifies the signal andapplies ments requires a degree of precision which mm the product o theControl grid of e tube to ders their use uneconomic. reduce itsconduction. An additional feature of It is an object of this invention,the fo t the system is the utilization of a resistor in series providean A, V. C. system wherein a greater (3011- with the cathode of thecontrol tube, to obtain trol effect is obtained with a minimum of inaself-bias which delays diode rectification until stability introducedthereby. a desired minimum signal level is exceeded, and

It is a further bje t of thi i ti t pro- 40 stabilizes the maximumcurrent through the control tube. 1

Referring now more particularly to the drawings, the last intermediatefrequency stage of a receiver is indicated by a circuit designated byCoupled thereto is a detector stage II of the receiver, from whichautomatic volume control signal input is obtained through a condenserl6. A-combination diodetriode control tube I4 is provided, the triodepor tion having its anode 20 connected to a source of positive potentialand its cathode l9 connected to the automatic volume control bus l3.

The bus [3 has applied. thereto a potential which is negative withrespect to the cathodes of the tubes controlled thereby and probably tothe chassis in general, and of suificient magnitude to reduce the gainof the receiver to a value suitable for the reception of the strongestsignal apt to be encountered. This potential is obtained from a sourceof relatively high impedance, such as an existing diode section of atube rectifying an al ternating potential from the primary source ofpower; from a local oscillator; or through a high resistance from anormally rectifying tube element, such as the control grid of anoscillator or mixer. The latter type of source is shown here, the bus l3being shown as connected to the control grid 29 of an oscillator itthrough ahigh re sistance 24. A condenser 25 is connected between thebus 13 and ground for filtering purposes,

The resistance 24 is of sufficient magnitude so that a relatively smallcurrent passing through the normally conducting triode section of thecontrol tube M- will serve to nullify the negative potential applied tothe bus [3 in the absence of a signal suflciently strong :to operate thecontrol tube.

The utilization of a high impedance source for this purpose avoids thenecessity of adding separate rectifying and filtering means to supplythis voltage, thus reducing the expense of automatic volume control.

:Signa l voltage for volume control urposes is obtained fromtl1e.detec'to1-stage H through re sister 2! and is applied to a circuitof which the diode section of tube i i forms a part and which will nowbe described.

The anode l8 of this section is connected to the cathode 19 through aresistor 22, and to the grid ill through a resistor 2!. A condenser 23is connected between the grid ll and the bus 53. Signal voltage isapplied to this circuit across the resistor 22 and :is rectified :by thecircuit comprising the anode l3, resistor 22 and cathode H9. Thenegative voltage Which is :a product of this rectification is applied tothe grid ll through the resistor i, the condenser 23 acting as afiltering :mcans.

By this .action the space current flow through the tube it will bereduced in accordance with signal strength and the nullifying actionexerted by the space current flow, With regard to the negative potentialapplied to the bus l3, will be varied in an inverse manner. The level ofnegative voltage available .on the bus 13 for volume I control will thusvary in accordance with signal strength.

The potential of the bus 13 is prevented from becoming positive inpolarity by the use of a diode 26 as a by-pass to ground.

Figure 2 illustrates a modification of the circuit of Figure 1 in whicha self-biasing means con- .sisting of a resistor 2"! shunted bycondenser 28 is inserted in the cathode circuit of the tube It. Theresistor 2"! generates a limited amount of self-bias due to spacecurrent flow during periods when no signal voltage is being applied tothe circuit, thus in effect applying a minimum signal amplitude delayvoltage to the tube without mak- 'ing any demand up :1 signal power. Aslong as the bias supplied by the resistor 21, between grid I? andcathode it, is not exceeded in amplitude by the amplitude of the signal,no rectification will take place and thus no work will be eliected bythe signal.

When the signal amplitude exceeds the initial delay bias developed acrosthe cathode resistor 21, rectification through the diode elements IS andI8 ensues and the bias developed across re- .sistor 22 is applied to thecontrol grid ll of thetriode section as before. However, as the triodespace current is reduced by this action, the delay bias is progressivelyreduced and the dinerence between the delay bias and the signalamplitude is increased, so as to provide an enhanced bias for thecontrol grid ll. If the initial bias developed across resistor 21 is afractional part m/n of the :cutoif bias of the triode grid l1, no A. V.C. potential isdeve'loped until the signal at the diode exceeds thispotential, and full A. V. C. potential is developed when the signalattains an amplitude sufiicient to develop the cutoff bias of the triodegrid H. The ratio of the maximum signal to the minimum signal capable ofdeveloping A. V. C. potential is then n/m, as seen at the dioderectifiers. By suitable design all useful signals may be made to fall inthis range, and the range may be made as small as may be desired.

In addition to the establishment of a delay voltage the resistor 2! actsto stabilize the maxi- :mum current flow through the control tube bymeans of the generated bias.

While the invention has been disclosed with reference to the embodimentsthereof which are now preferred, many changes and modifications fallingwithin the scope of the appended claims will occur to those-skilled inthe art. It is therefore to be understood that the invention is not tobe considered as limited to the embodiments disclosed herein.

What is claimed is:

1. In a system for the automatic volume control of a radio receiver: .acontrol channel; a source of negative potential of a magnitudesuflicient to provide the maximum desired control action; means applyingsaid negative potentialto said channel; a source of positive potential;a control tube comprising an anode, a cathode, and a control grid, saidcathode being connected to said control channel and said anode beingconnected to said source of positive potential; said source of negativepotential presenting to said control channel an impedance of such valuethat normal space current flow through said control tube in thequiescent state thereof will substantialy nullify the application ofsaid negative potential to said channel; a second anode in said controltube; a rectifying circuit; means for applying a signal voltage derivedfrom said receiver to said rectifying circuit, said rectifying circuitincluding said second anode and said cathode; means applying the averageproduct of said rectification to said grid; and self-biasing means forsaid control tube comprising a resistor in the circuit of said cathodeconnected between said cathode and said channel, and a condensershunting said resistor, said biasing means establishing a bias on saidgrid and second anode with respect to said cathode due to gridcontrolled space current fiow and rectification in said control tube.

'2. In a system for the automatic volume control of a radio receiver: acontrol channel; a voltage source applying to said control channel anegative potential of a magnitude sufficient to provide the maximumdesired control action; a source of positive potential; a control tubeconnected between said source of positive potential and said channel,the first mentioned source presenting to said channel an impedance ofsuch value that normal current flow through said tube in the quiescentstate thereof will substantially nullify the application of saidnegative potential to said channel; a control grid and a cathode-in saidtube; a rectifying circuit; means applying a signal voltage from saidreceiver to said rectifying circuit; means applying the average outputof said rectification to said grid; and resistance-capacitance meansconnected in series with the path of the total space current of saidtube between said cathode and said channel, said resistance-capacitancemeans applying a bias to said grid and rectifying circuit in response togrid controlled space current flow in said control tube and to theaction of said rectifying circuit.

3. In a system for the automatic volume control of a radio receiver: acontrol channel; a source of negative potential of a magnitudesufilcient to provide the maximum desired control action; means applyingsaid negative potential to said channel; a source of positive potential;a control tube comprising an anode, a cathode, and a control grid, saidcathode being connected to said control channel and said anode beingconnected to said source of positive potential, said source of negativepotential presenting to said control channel an impedance of such valuethat normal space current flow through said control tube in thequiescent state thereof will substantially nullify the application ofsaid negative po- REFERENCES CITED The following references are ofrecord in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,207,094 Getaz July 9, 19402,207,905 Weagant July 16, 1940 2,255,683 Singer Sept. 9, 1941 2,390,856Thompson Dec. 11, 1945 2,434,929 Holland Jan. 27, 1948

